1. Field of the Invention
This Application relates to Bucky-type grids used in x-ray radiology, and more particularly to an improved scanning grid apparatus exhibiting high primary and low secondary or scattered radiation transmission.
2. Description of the Prior Art
A conventional Bucky grid consists of an array of radiopaque (usually lead) foil strips which are separated by strips of radiolucent spacing material. The array is positioned between an object being irradiated and an image receptor so that the image-forming x-rays from the focal spot of the x-ray source see only the edges of the foil strips and the majority pass through the radiolucent spacers. A significant portion (typically 30-45%), however, of the image-forming or primary x-rays are attenuated by the lead strips and interspacing material. Scattered x-rays are omitted, on the other hand, from the patient in all directions and the majority of these that are emitted toward the image receptor do not have a straight line path through the radiolucent spacers to the image receptor and are therefore absorbed (typically 85-95%) in the lead.
Typically, the thickness of lead foil strips is 2 mils and the thickness of interspace material varies from 8 to 15 mils, depending on the number of grid strips or lines per inch. The geometry of the conventional grid is depicted in FIG. 1. The ratio of the height (h) of the lead strips to the interspace material thickness (D) or h/D is defined in the literature as the grid ratio (r). Typical grid ratio values vary from 4/1 to 16/1. For a given number of grid lines per inch as one increases the height (h) of the lead strips or grid ratio, the scattered radiation transmitted by the grid decreases. However, two factors limit this in practice. These factors are the increased attenuation by the interspace material (usually cotton fiber or aluminum) of the primary X-Ray, i.e. non-scattered, beam and increased difficulty in aligning and maintaining alignment of the grid with the primary X-Ray beam.
The lead strip arrangements commonly employed in different types of grids are illustrated in FIGS. 1 and 2 with the linear focused grid being the most widely used. The advantage of a focused grid compared to a linear parallel grid is that there is no grid cut-off at the edges of the grid as there is with a parallel grid. Grid cut-off is a loss of primary radiation resulting from the primary X-Rays not being properly aligned with the lead strips. Another type of grid that is occasionally employed, that is not depicted in FIGS. 1 and 2, is a crossed grid. It is made up of two orthogonal superimposed linear grids which have the same focus distance.
One disadvantage of using a grid is that a pattern of clear (white) lines is cast onto the film. This pattern of grid lines can be distracting to the radiologist, especially if the lines are thick and widely spaced. The common solution to this problem is to move the grid sideways (perpendicular to the grid strips) across the film during an exposure. The shadows of the grid strips are blurred out and are, therefore, not visible. However, this blurring movement results in decreased primary transmission, since the x-ray source cannot remain aligned with the focal line of the grid while it is being moved. This decrease in primary transmission associated with the grid blurring movement is more pronounced for high ratio grids.
Since the issuance of the original Bucky patent (U.S. Pat. No. 1,164,987), further scatter suppression techniques have been disclosed in the patent literature, as, for example, evidenced in U.S. Pat. No. 1,484,663 to Mutscheller, and the recent U.S. Pat. No. 4,126,786 to LeMay et al. Mutscheller discloses an x-ray filter for preventing secondary x-rays from reaching an object to be operated upon by primary x-rays. His filter includes, among other things, a grid and means for moving the grid bodily while in action, the grid being provided with slats each journaled to turn slightly upon an axis of its own, and to provide means for thus turning the slats, in order to maintain each slat exactly edgewise relatively to the position of the target, and for coordinating the turning movement of the slats with the bodily movement of the grid. LeMay et al discloses a computerized axial tomography (CAT) scanner of the type in which a fan of radiation affects a rotational scan and the detectors are fixedly positioned, wherein post-patient collimation is achieved by means of plural plates hingedly attached adjacent the detectors for defining primary x-ray paths to the detectors and shielding the detectors from scattered radiation. The LeMay et al post-patient collimator is tiltable in the plane of the patient's radiation slice to permit radiation, originating from a range of positions to impinge upon the detectors. According to LeMay et al, the collimator is constructed of a plurality of plate members protruding from the detectors towards the patient position, wherein a pair of the plate members flank each individual detector and wherein the plate members are hinged at their ends adjacent the detectors to permit tilting to occur.
In French Pat. No. 778,803 is disclosed a secondary radiation absorbing grid in which the individual grid members are implemented by means of a variety of zig-zag, wavy, and/or crinkle grid septa shapes. See also Brezovich and Barnes, "A New Type of Grid", MED. PHYS. 4:451-453, September/October 1977.